This book discusses soy protein nanoparticle-based polymer blends, composites and nanoconposites aling with their chemistry, processing, preparation, characterization, applications as well as the soy protein-based materials rheology.
After discussing the preparation of soy protein nanoparticles, the characterization methods such as atomic force microscope (AFM), transmission electron microscope (TEM) and scanning electron microscope (SEM), for the nanoscale soy protein reinforcements are examined. The various processing methods for nanocomposites and their mechanical, thermal properties, barrier properties are then discussed. The book then moves on to discussing the different types of blends, composites and nanocomposites with their type of polymer matrixes such as thermoplastics, thermoset, natural rubbers and synthetic rubbers.
Soy Protein: State-of-the-Art, New Challenges and Opportunities
Visakh P. M
Department of Ecology and Basic Safety, Tomsk Polytechnic University, Tomsk, Russia
Corresponding author: email@example.com
This chapter deals with a brief account on various topics in rubber-based bionanocomposites: Preparation and state-of-the-art. It also discusses different topics such as soy protein: Introduction, structure and properties relationship, thermoplastic-based soy protein nanocomposites, applications of soy protein-based blends, composites, and nanocomposites, biomedical application of soy protein, preparation of soy protein nanofibers by electrospinning, physiologically active peptides derived from soy protein, soy protein polymer-based (film) membranes and encapsulation of bio actives using soy protein-based material.
Keywords: Rubber-based bionanocomposites, soy protein, soy protein nanocomposites, soy protein nanofibers
1.1 Soy Protein: Introduction, Structure and Properties Relationship
Soy proteins are one of the most abundant and most widely utilized plant proteins on this planet. With high content of essential amino acid and desirable functional properties, soy proteins have attracted persisting interest in food and pharmaceutical industry. The 11S and 7S globulins represent approximately 60% of the storage protein in soybeans. They are the most important contributors to the physicochemical and functional properties of soy protein products. It exhibits a high content of negatively charged amino acids such as glutamic acid and aspartic acid, whereas the percentage of hydrophobic amino acids such as leucine is relatively low . ß-conglycinin is relatively flexible, as evidenced by its high contents of a-helix, and random coils . It comprises six major isomers, each of which is composed of three major subunits and two minor ones (? and d) .
As can be seen, the whole soybean seed is cleaned, cracked, dehulled, and flaked to produce soy powder. The powder is then subjected to oil extraction with organic solvents such as hexane. The particle sizes range from grits (or flakes) of varying sieve specification to fine powders. The soy meal could be further ground into soy flour (SF), a product that contains less than 1% oil and a protein content ranging from 40-60%. Soy proteins with higher purity may also be produced with smaller particles, because the protein can be more effectively extracted from finer flours, making the separation of protein from insoluble carbohydrate more efficient and complete.
There exist three major types of soy protein-rich products, SPC (soy protein concentrate, and fractionated 11S/7S globulins (protein content >90%, fraction purity >85%). Quite a few methods have been developed to produce these products with desirable features, and several typical approaches with respect to their principles, major procedures, advantages, and drawbacks. The majority of the protein is precipitated and recovered by a second centrifugation. The curd-like precipitate is neutralized with alkali, washed with water to remove excessive alkali and salt, and finally spray dried or lyophilized to yield the final product. The typical protein of yield (weight ratio between the product and the raw material) is around 30%, though a yield of as high as 44% has been reported . The product loses part of its original solubility as a result, but it gains some desirable properties such as good texture and water holding capacity .
In pilot scale production, the solvent such as alcohol and hot water can be recovered through evaporation and condensation, thus achieving higher extraction efficiency. In addition to the traditional methods, membrane-based techniques including micro- and ultrafiltration have also been widely studied for the preparation of SPC. Teng et al. further investigated the effect of divalent cations on the fractionation process. They suggested that using Mg2+ instead of Ca2+ as a precipitant improved the purities of both fractions without affecting their yields significantly. Soy proteins tend to adopt a compactly folded structure, with their hydrophilic and charged amino acid residues maximally exposed to the solvent and hydrophobic moieties buried in the globular core. The surface charge of colloidal particles is usually gauged by the electrical potential at the interfacial double layer at the location of the slipping plane relative to a point in the bulk fluid away from the interface. Proteins as amphiphilic molecules bear both hydrophilic and hydrophobic groups which endow their ability to interact with both the polar and nonpolar solvents and serve as an emulsifier .
Two parameters are commonly referred to when describing the emulsifying properties of a molecule. As many other proteins, soy proteins show viscoelasticity when dispersed in water. Under room temperature without the addition of cross-linkers (such as transglutaminase or calcium salts), the dispersion exhibits viscous property (G?) as the predominant characteristic . Since viscosity is indicative for the friction between the molecule and the solvent, it is highly dependent on the interaction between them. Heated soy protein films exhibit decreased water vapor permeability, and increased percentage of elongation at break (%E) when compared to unheated ones .
While thermal denaturation is conventionally considered as a detrimental factor for protein solubility, combination of thermal treatment with a suitable pressure may make the protein more soluble. Glycerol is by now the most widely utilized plasticizer for soy protein-based plastics, owing to its relatively short and flexible chain as well as its strong hydrophilicity. The former character facilitates the insertion of glycerol into the peptide chains in the soy proteins, and the latter one promotes its interaction with the protein via extensive hydrogen bonding. Soy proteins are rich in both amine and carboxyl groups; therefore, they can readily react with additional carboxyl or amine groups. The reaction between the positively charged amine groups on the soy proteins and an external carboxylic acid is comparable to phosphorylation.
1.2 Advances in Soy Protein-Based Nanocomposites
Residual soy proteins, a by-product of the soy oil industry, are currently utilized in applications such as animal feed and food supplement. Soy proteins are composed of a mixture of albumins and globulins, 90% of which are storage proteins with globular structure, consisting mainly 7S (conglycinin) and 11S (glycinin) globulins. Soy protein contains 18 amino acids including those containing polar functional groups, such as carboxyl, amine, and hydroxyl groups that are capable of chemically reacting and making soy protein easily modification . Biopolymer films are usually plasticized by hydroxyl compounds . Glycerol has a high boiling point and good stability, and is regarded as one of the most efficient plasticizers for soy protein plastics . Glycerol-plasticized soy protein possesses good processing properties and mechanical performance . The bio-nanocomposites consist of a biopolymer matrix reinforced with particles having at least one dimension in the nanometer range (1-100 nm) and exhibit much improved properties due to high aspect ratio and high surface area of the nanoparticles.
Soy protein films reinforced with starch nanocrystals (SNC) could be prepared by casting method . The SNC synthesis was developed by acid hydrolysis of native cornstarch. Soy protein is one of the few natural polymers that can be thermoplastically processed under the plasticization of small molecules . Soy protein plastics without any additive have a brittle behavior, which makes processing difficult. Addition of plasticizers is an effective way to improve the flowability of soy protein melts and obtain flexible soy protein-based films. Phthalic anhydride modified soy protein (PAS)/glycerol plasticized soy protein (GPS) composite films were fabricated by using extrusion and compression-moulding . Soy/BN nanocomposites were prepared by low-cost green technique with water as the solvent. The thermal properties of the nanocomposites were studied by thermogravimetric analysis (TGA). The biodegradation behaviors of maleated PCL/ isolated soy protein (SPI) composites reinforced with organoclay were evaluated by soil burial test . Composites containing higher percentage of soy protein degraded rapidly in the initial 8 weeks and a gradual decrease of weight occurred during the next 8 weeks.
Soy protein films are effective barriers to the passage of lipid, oxygen, and carbon dioxide. However, the inherent hydrophilicity of proteins and the substantial amount of plasticizer added in the film perform poorly in moisture barrier and mechanical properties as packaging material. In addition of in situ synthesis, soy protein/silica nanocomposites could be fabricated through compounding nano-SiO2 particles into soy protein isolate matrix . Zheng et al. reported the nanocomposite sheets by compounding MWNTs of various sizes into SPI matrix through solution mixing and then compression-molding method . Blending SPI with other biodegradable polymers such as polycaprolactone, poly(lactic acid), poly(vinyl alcohol), natural rubber, etc., thus becomes a way to enlarge its applications. The properties of the blend materials could further improved by nanoreinforcing. Sasmal et al. prepared a kind of bio-based, eco-friendly nanocomposites from maleated polycaprolactone/soy protein isolate...